To participate in road traffic with a trailer connected to a drawing vehicle, the trailer must be equipped with lighting according to regulation. Therefore, vehicle trailers have an electric system to which several different lamps are connected. Usually, a RH and a LH tail lamp, a RH and a LH direction indicator and stop lights are compulsory. In addition, a back-up light and a rear fog lamp can be installed. It must be possible to control the different lamps from the towing vehicle. Usually, this is achieved by connecting each lamp of the trailer with an individual line to the control unit in the drawing vehicle. The stop lamps are connected to the control unit by means of a common line. These lines are led through a plug-connection between the drawing vehicle and the trailer. On the drawing vehicle side, the trailer connection box forming part of the plug-connection is provided, which interacts with a connector forming part of the plug connection on the trailer side. In the trailer connection box as well as in the connector, contacts are provided by means of which the sections of the lines between the control unit and the connection box on one hand and between the connector and the lamps on the other hand can be connected.
A device for the monitoring of a trailer connection box of the type initially mentioned known to the applicant comprises several circuits having the following layout:
Each circuit has
a first connection for the connection with a positive potential of a voltage of a vehicle electric system,
a second connection for the connection with a contact of the trailer connection box,
a measuring resistor being arranged between the first and the second contact,
a first voltage sensor and a first voltage divider being equipped for the voltage measurement on a first side of the measuring resistor,
a second voltage sensor and a second voltage divider being equipped for the voltage measurement on a second side of the measuring resistor, and
a controllable switch connected in series with the measuring resistor.
These circuits of the device known to the applicant are on one hand connected to the positive potential of a voltage of a vehicle electric system and on the other hand to a contact of the trailer connection box. As modern passenger vehicle trailers have a maximum of seven trailer lamps (in this context the stop lamps count as one lamp as they are connected to the control unit in the drawing vehicle via a common line), there must also be at least seven contacts in the connection box. These seven contacts of the connecting box must/can be monitored. This is why in the device known to the applicant there are up to seven circuits of the type described above. In older trailer variants an adapter can reduce the 13-pin trailer connection box (according to DIN) provided by the vehicle to a 7-pin connection (according to DIN) in which not all seven light lines are supported. In this case the light lines which cannot be monitored are recognized as not existing.
Even if for the detection whether a trailer is connected to the drawing vehicle or a trailer is electrically connected to the drawing vehicle only four lines resp. four contacts are required for measurement, only four of the above circuits are required for the trailer detection. Monitoring of all lamps, however, is not possible in a device with four circuits of the above type.
Due to the large number especially of active components, all devices of the type known to the applicant are very complex. Furthermore, measurements in a circuit with two voltage sensors are time-consuming and require a major computational effort for the evaluation of the measurements. Also, due to the large number of components needed for measurement, the number of measurement errors rises. These may be negatively influenced by temperature drift and aging in addition. Deviations due to analog/digital conversions also contribute to measurement deviations.